Almost 20 million children age 6-18 years are overweight or obese, and over 40% of Latino and African- American youth are affected. The public health impact of obesity is enormous, with health care costs alone exceeding $100 billion dollars annually. In response to this public health crisis, the Institute of Medicine has recommended that schools institute body mass index (BMI) screening and report results to parents. However, it is unclear if this practice can actually reduce rates of obesity. There is a critical gap in our knowledge for three reasons. First, half of the states now direct limited schoo resources to BMI screening and reporting, and implementation has not been uniform - some states report only BMI to parents, while others combine BMI with comprehensive fitness test results, with no studies of the comparative benefits of these approaches. Second, studies on the effect of BMI screening and reporting have reached different conclusions. Arkansas saw a plateau in childhood obesity rates after implementing BMI reporting in schools, but Arkansas implemented multiple obesity-related interventions simultaneously. Our natural experiment showed no effect of BMI reporting in California, but was limited by suboptimal BMI reporting methods. However, small studies have shown some promise for BMI reporting to reduce obesity rates, particularly among minority youth. Finally, experts have raised concerns that assessing BMI in the school setting and reporting results to parents may increase stigmatization of overweight students, yet studies have not adequately addressed these concerns. Our objective is to provide a rational basis for further policy development, via three Specific Aims: 1) to provide evidence on BMI screening and reporting's impact on obesity and obesity disparities; 2) to determine the most effective format for reporting BMI results to parents; and 3) to determine the extent to which school- based BMI screening and reporting increase weight-based stigmatization. To achieve these Aims we will conduct a 3-year cluster-randomized trial of BMI screening and reporting, randomizing 75 California schools that do not currently report BMI to parents to 1 of 3 groups: 1) BMI screening and reporting; 2) BMI screening only; or 3) neither BMI screening nor reporting. Within arm 1 (BMI screening and reporting), we will randomize students to receive one of two BMI reports: BMI results only or BMI with comprehensive fitness results. This proposal is significant because it addresses essential gaps that have implications for youth across the nation: does the now widespread practice of BMI screening and reporting actually reduce obesity rates or disparities? And, do the two widely implemented BMI report formats - providing BMI alone vs. BMI with fitness results - differentially impact obesity and fitness? This proposal, as the first large-scale RCT of BMI screening, is innovative because it simultaneously examines BMI screening and reporting's potential benefits (obesity reduction) and risks (weight-based stigmatization). Without this research, we cannot have fully informed discussions about school-based policies to address childhood obesity.